1. Field of the Invention
The present invention is generally directed to medical imaging. More particularly, the present invention is directed to advanced image fusion systems and methods for use in image-assisted biopsy, image-assisted treatment planning, image-assisted treatment and image-assisted surgery, and molecular imaging.
2. Description of the Related Art
Nuclear medicine imaging tools, such as single-photon-emission-computed-tomography (SPECT) and positron-emission-tomography (PET) are known. Other types of medical imaging tools, such as magnetic resonance imaging (MRI), computed tomography (CT) or ultrasound (US), are also well known. Each of these known imaging techniques (modalities) provides a clinician with a different type of data and is useful for a different purpose, such as mainly functional (SPECT, PET) or mainly anatomical imaging (MR, CT, US). Until recently, images from only two modalities were combined in one display image. Combining these images usually requires two processes: 1) co-registration and 2) Image Fusion.
A medical imaging procedure typically creates a volume or matrix of measured values that forms the basis for that modality's image of the patient. Each value in the matrix represents image data at the geometric center of a voxel (volume element) of the volume. Co-registration is a process where the voxels in each of the images obtained by different modalities are transformed to correspond to a common reference frame. This can be done either via a rigid body or non-rigid body (“elastic”) transformation of the pixel positions. In a rigid body transformation, only scaling, translations, and rotations are allowed, keeping the relative positions between all voxels constant. In an elastic transformation, additional to the fore-mentioned rigid body transformation, the voxel positions may also be elastically deformed, and relative distance between all voxels in the input image positions does not have to be preserved.
Image Fusion (IF) is a technique to display co-registered images obtained from two different imaging devices. IF can be characterized by the following: Let Ii denote an image volume of modality Mi. In a previous step, separate image volumes from different devices are co-registered, in order to have image values at common pixel positions {right arrow over (rj)}={right arrow over (rj1)}={right arrow over (rj2)},∀j, with pixel index j within the coregistered volume. In the IF technique, the fused image I3 of images I1 and I2 is displayed on a display device with the display-pixel color and intensity determined from a function of the coregistered image values of I1 and I2. The function is often, but not limited to a linear combination of the image pixel value in each image (“alpha-blending”), which can be easily mathematically represented by:Ii=Ii({right arrow over (rj)})∀j; I3=c1I1+c2I2;  (Eq. 1)In general: I3=f(I1, I2), where f may be any function combining the 2 images, however all pixels are involved.
By this technique, the images from two different devices or modalities (1 and 2) are displayed simultaneously in the same region in space, if the volumes are accurately co-registered. With current techniques, the entire image volumes of two images are fused using constant coefficients c1 and c2. The color table lookup index, T, is derived from I3.
Current techniques are known to work well if images with similar resolution and noise characteristics are fused, or if the image information is equally distributed throughout the image volume in all images to be fused. However, if the resolution and noise characteristics are quite different, as is the case, for example, with functional and anatomical imaging, such as SPECT and CT images or MRI images, then the information delivered to an observer can be less than optimal, and more difficult to interpret with a simple IF technique. For example, referring to FIG. 1, the SPECT image of a prostate is fused with a CT image of the same prostate by the current IF technique. One can see that the critical information for the prostate in the center of the resulting image 100 is hard to read, because the entire SPECT image with its high noise and low spatial resolution is superimposed over the low-noise and high-spatial resolution CT image. In this example, the information of interest in the SPECT image is a small region of local uptake (“Region of Interest”) corresponding to the functioning of the prostate, while the anatomical relevant information in the CT image is generally distributed throughout the entire image slice.
Current methods for combining images also fail to address the need of multi-modality imaging for quantitative use, treatment planning and monitoring, for systems with which more than two modality volumes could be registered, such as Ultrasound or CT, and SPECT/PET.
Another method to display co-registered images, shown in FIG. 3, provides two images from different devices or different modalities side by side. Here, areas of the images can be correlated by manipulating a correlated cursor 302, which points to the same region on each view. This method is also not optimal because the images are not overlaid at all, and the clinician is forced to make assumptions and estimations by visually comparing two separate images, with only the correlated cursor as an aid.
Accordingly, there is a need for new and improved systems and methods of combining image volumes from disparate imaging devices, to enhance clinical results and/or image-assisted biopsy, image-assisted treatment planning, image-assisted treatment and image-assisted surgery, and molecular imaging.